whom 在从句中作宾语,所以此句是正确的.
whom I think you have known为定语从句,i think是插入语,可不看.从句主语是you,have known 为从句谓语,whom 指代先行词actress,在从句中作宾语.
如果要换成which,最好在actress 后加个逗号.
Lucy is a successful Asian-American actress,which I think you have known.
此时which 指代“Lucy is a successful Asian-American actress ”一整个句子.